This invention relates to a magnetoresistive head, and in particular to a magnetoresistive head which is used for reproduction of information recorded in magnetic recording medium such as magnetic tape, etc.
A known magnetoresistive head (hereinbelow abbreviated to MR head) has, as indicated in FIG. 1, a structure such that a first insulating layer 2 is formed on a first block 1 made of a magnetic material such as ferrite, etc.; then a bias conductor 3 and a magnetoresistive element (hereinbelow abbreviated to MR element) 4 are formed; similarly a second insulating layer 5 is formed on a second block 6 made of a magnetic substance such as ferrite, etc.; and these are joined, as disclosed e.g., in JP-B-53-25646 corresponding to JP-A-49-74523 laid-open on Jul. 18, 1974 and corresponding to U.S. Ser. No. 296742 filed on Oct. 11, 1972. In FIG. 1 they are indicated, for the sake of clarity, in a view exploded at the central portion. The two blocks 1 and 6 serve as a magnetic shield which protect the MR element and at the same time remove unnecessary signal magnetic field so to increase the resolving power. Further, on the MR element 4, there is closely disposed the bias conductor 3 which applies a suitable bias magnetic field thereto so that the variation thereof in the electrical resistance is optimum.
It is known that the variation in electrical resistance .DELTA.R of the MR element when an external magnetic field H is applied to the MR head (hereinbelow called .DELTA.R-H characteristics) is as indicated by a curve in FIG. 2. Since, in response to the signal magnetic field of a recording magnetic medium such as magnetic tape, etc., the electrical resistance varies from the resistance resulting from the bias magnetic field H.sub.B applied by the bias conductor 3, depending on the intensity of the signal magnetic field. The variation of the resistance is taken out as a reproduction output (voltage) by making a predetermined operation current flow through the MR element 4. Consequently, the reproduced output varies, depending on the MR element 4 and the intensity of the current flowing through the bias conductor 3. As indicated in FIG. 3, the reproduction output generally increases proportionally to the operation current I flowing through the MR element 4. However, the reproduction output is saturated because of heat generation, etc. for current intensities higher than a certain value, and it decreases, even if the current is increased thereabove, as indicated by the characteristic curve in FIG. 3.
Furthermore, it can be understood that, when the .DELTA.R-H characteristics indicated by the solid line in FIG. 2 are changed to those indicated by the broken line, and since the variation .DELTA.R in electric resistance with respect to the signal magnetic field varies, the curve representing the relation between the operation current and the reproduction output (hereinbelow called sense current characteristics) indicated in FIG. 3 also varies. The relation between the magnetostriction constant of the MR element 4 and the stress applied to the heat during the fabrication thereof may be cited as one of the factors causing the changes of the sense current characteristics. This is because the magnetic characteristics of the MR element 4 vary because stress is produced by strain generated by heating or cooling during the steps of film formation because of differences in the Young's modulus and the thermal expansion coefficient among the constituent materials thereof, or by strain generated by variations in the shape of the block taking place at the junction of the electromagnetic conversion gap, etc.
An invention disclosing control of the electromagnetic conversion characteristics by using the stress, taking into account the magnetostriction constant, of the MR element, is disclosed in JP-A-60-205814. The structure of the MR head is basically identical to that indicated in FIG. 1. According to the known techniques, stress is applied from the exterior to the MR head so that the electromagnetic conversion characteristics are optimum, depending on the magnetostriction constant of the MR element when the MR head is mounted on a fixing plate for holding the MR head.